The magneto-optical recording method is a method for recording and reproducing, in a manner described below, to and from a recording medium composed of a substrate and a perpendicularly-magnetized film made of a magnetic substance which is formed on the substrate.
The recording operation begins with initialization of the recording medium by a strong external magnetic field or the like, whereby the magnetization of the recording medium is directed in one specific direction (either upward or downward). Thereafter, a laser beam is projected on an area where information is to be recorded, so that the area of the recording medium is heated to not lower than around a Curie temperature of the magnetic film or not lower than around a compensation temperature of the magnetic film. As a result, the heated area of the magnetic film has no coercive force (Hc), or substantially no coercive force (Hc). In this state, an external magnetic field (bias magnetic field) with a magnetization direction opposite to that of the magnetization for the initialization is applied so that the magnetization direction of the area is reversed. When the projection of the laser beam is suspended, the temperature of the recording medium falls to the room temperature, whereby the magnetization thus reversed is fixed. Thus, information is thermal-magnetically recorded.
For reproducing information, a linearly polarized laser beam is projected on the recording medium, so that optical reading-out of information is carried out by making use of a phenomenon that the plane of polarization of reflected light or transmitted light of the laser beam rotates in accordance with the magnetization direction (the magnetic Kerr effect, and the magnetic Faraday effect).
The magneto-optical recording method has been viewed with interest as a method for recording with respect to rewritable high density and large capacity memory device. As such a method for reusing (rewriting) the media, a so-called light modulation overwriting method has been proposed. According to this method, overwriting by the light intensity modulation is carried out with respect to a recording medium which has a recording layer composed of two exchange-coupled films, by using an initializing magnetic field (Hi) and a recording magnetic field (Hw).
An magneto-optical recording medium to which the light modulation overwriting method is applicable is proposed in the Japanese Examined Patent Publication 5-22303/1993. As shown in FIG. 8, a recording layer of the recording medium disclosed in the publication is triplicated with a second magnetic layer 14 being provided between a first magnetic layer 13 and a third magnetic layer 15, so that the initializing magnetic field (Hi) is allowed to be smaller and that the recording medium has superiority in the stability of recording bits. The following description will depict steps for overwriting the described recording medium.
FIG. 9 is a view illustrating states of respective magnetizations of the first through third magnetic layers 13 through 15, wherein the horizontal axis indicates temperature. Since the layers are rare earth-transition metal alloys, each has a total magnetization and respective sub-lattice magnetizations of rare-earth and transition metal. Voided arrows represent the directions of the transition metal sub-lattice magnetizations of the respective layers.
Initialization is carried out by applying the initializing magnetic field Hi so that, as shown in FIG. 9, only the magnetization direction of the third magnetic layer 15 is directed in one specific direction (upward in the figure). Since a strength of the initializing magnetic field Hi is smaller than that of a coercive force of the first magnetic layer 15 at room temperature while greater than that of a coercive force of the third magnetic layer 15 at room temperature, the magnetization direction of the first magnetic layer 13 is not reversed. The second magnetic layer 14 has an in-plane magnetic anisotropy at room temperature. Therefore, it has an effect of preventing exchange-coupling between the first magnetic layer 13 and the third magnetic layer 15.
Recording is carried out by applying the recording magnetic field Hw while projecting the laser beam whose light intensity is modulated either to a high power or a low power.
The high power of the laser beam is set so that the projection of the high power laser beam causes the recording medium to be heated to the vicinity of a Curie temperature of the third magnetic layer 15. The low power of the laser beam is set so that the projection of the low power laser beam causes the recording medium to be heated to the vicinity of a Curie temperature of the first magnetic layer 13.
Therefore, on the projection of the high power laser beam, the magnetization direction of the third magnetic layer 15 is reversed downward as shown in FIG. 9, by the recording magnetic field Hw. The magnetization direction of the third magnetic layer 15 is copied, by the exchange coupling force exerted to an interface in the cooling process, to the second magnetic layer 14 having a perpendicular magnetic anisotropy, and then to the first magnetic layer 13. As a result, the magnetization direction of the first magnetic layer 13 is directed upward.
On the other hand, the magnetization direction of the third magnetic layer 15 is not reversed on the projection of the low power laser beam, since in such a state a strength of the coercive force of the third magnetic layer 15 is greater than that of the recording magnetic field Hw. The magnetization direction of the first magnetic layer 13 is directed in the same direction as that of the magnetization of the third magnetic layer 15 by the exchange-coupling force exerted to the interface in the cooling process, as described above. Therefore, the magnetization of the first magnetic field 13 has a downward direction as shown in FIG. 9.
Note that FIG. 10 illustrates, about a conventional magneto-optical recording meidum, a relation between a perpendicular magnetic anisotropy Ku3 at room temperature of the third magnetic layer 15 of the conventional magneto-optical recording medium and the recording magnetic field Hw, and a relation between the perpendicular magnetic anisotropy Ku3 and the initializing magnetic field Hi. As shown in the figure, the recording magnetic field Hw is set considerably smaller than the initializing magnetic field Hi. A reproduction-use laser power is set considerably smaller than the recording-use low laser power.
The above-described conventional technology thus has provided a magneto-optical recording medium (1) to which the light modulation overwriting method is applicable, (2) which allows an initializing magnetic field to be relatively small, and (3) which is superior in the stability of the recording bits. The technology, however, has still presented a problem that the initializing magnetic field Hi greater than the recording magnetic field Hw is required, which leads to a problem that apparatuses cannot be miniaturized.
Furthermore, in the case of a disk-type magneto-optical recording medium, an apparatus which can generate such a great initializing magnetic field to be used with the disk-type magneto-optical recording medium does not meet the standards of the International Organization for Standization (ISO). Therefore, arises a problem that the foregoing conventional magneto-optical recording medium cannot be compatible with other recording-reproducing apparatuses in accordance with the ISO Standard.